exclusive locks, ugh

Annex/Content.hs

@@ -65,8 +65,8 @@ inAnnexSafe = inAnnex' $ \f -> openForLock f False >>= check
 		is_unlocked = Just True
 		is_missing = Just False
 
-{- Content is exclusively locked to indicate that it's in the process
- - of being removed. (If the content is not present, no locking is done.) -}
+{- Content is exclusively locked while running an action that might remove
+ - it. (If the content is not present, no locking is done.) -}
 lockContent :: Key -> Annex a -> Annex a
 lockContent key a = do
 	file <- fromRepo $ gitAnnexLocation key
@@ -85,11 +85,14 @@ openForLock file writelock = bracket_ prep cleanup $
 		(const $ return Nothing)
 	where
 		mode = if writelock then ReadWrite else ReadOnly
-		-- Since files are stored with the write bit disabled,
-		-- have to fiddle with permissions to open for an
-		-- exclusive lock.
-		prep = when writelock $ allowWrite file
-		cleanup = when writelock $ preventWrite file
+		{- Since files are stored with the write bit disabled,
+		 - have to fiddle with permissions to open for an
+		 - exclusive lock. flock locking would avoid this,
+		 - but -}
+		prep = forwritelock $ allowWrite file
+		cleanup = forwritelock $ preventWrite file
+		forwritelock a = 
+			when writelock $ whenM (doesFileExist file) $ a
 
 {- Calculates the relative path to use to link a file to a key. -}
 calcGitLink :: FilePath -> Key -> Annex FilePath

doc/bugs/cyclic_drop.mdwn

@@ -73,6 +73,15 @@ Another cycle might be running move --to and move --from on the same file,
 locally. The exclusivity of the content lock solves this, as only one can
 run at a time.
 
+Would it work with a shared lock? The --to would run git-annex-shell
+inannex. The --from would also be running, and would run git-annex-shell
+dropkey. So inannex and dropkey would end up running on the remote
+at the same time. Dropkey takes the content lock, and inannex checks it,
+but what if inannex runs first? Then it returns true, and then the content
+is removed, so both the --to and --from see success and the --to proceeds
+to remove the local content that the --from already caused to be removed
+from the remote. So, no, the nasty exclusive lock is needed.
+
 ---
 
 Another cycle might involve move --from and drop, both run on the same